1. Field of the Invention
The present invention relates to an optical image measuring apparatus employing a structure in which an object to be measured which is particularly a light scattering medium is irradiated with a light beam and a surface form or inner form of the object to be measured is measured based on a reflected light beam or a transmitted light beam to produce an image of a measured form. More particularly, the present invention relates to an optical image measuring apparatus for measuring the surface form or inner form of the object to be measured by using an optical heterodyne detection method to produce the image of the measured form.
2. Description of the Related Art
In recent years, attention has been given to optical imaging techniques for producing an image of a surface or inner portion of an object to be measured using a laser light source or the like. In contrast to the conventional X-ray CT technique, optical imaging technique is not hazardous to human bodies. Therefore, its application to the field of biomedical imaging is highly desired.
An example of a typical method of the optical imaging technique is a low coherent interference method (also called “optical coherent tomography” or the like). This method uses the low coherence of a broad band light source having a wide spectral width, such as a super luminescent diode (SLD). According to this method, reflection light from an object to be measured or light transmitting therethrough can be detected with a superior distance resolution of μm order (for example, see Naohiro Tanno, Kogaku (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
FIG. 6 shows a fundamental structure of a conventional optical image measuring apparatus based on a Michelson interferometer, serving as an example of an apparatus using the low coherent interference method. An optical image measuring apparatus 100 includes a wide band light source 101, a mirror 102, a beam splitter (half mirror) 103, and a photo detector 104. An object to be measured 105 is made of a scattering medium. A light beam from the broad band light source 101 is divided by the beam splitter 103 into two parts, that is, a reference light R propagating to the mirror 102 and a signal light S propagating to the object to be measured 105. The reference light R is a light beam reflected by the beam splitter 103. The signal light S is a light beam transmitting through the beam splitter 103.
Here, as shown in FIG. 6, the propagating direction of the signal light S is set as a z-axis and a plane orthogonal to the propagating direction of the signal light S is defined as an x-y plane. The mirror 102 is shiftable in both of the forward and backward direction, as indicated by a double-headed arrow in FIG. 6 (z-scanning direction).
The reference light R is subjected to a Doppler frequency shift by z-scanning when it is reflected on the mirror 102. On the other hand, the signal light S is reflected from the surface of the object to be measured 105 and from the inner layers thereof when the object to be measured 105 is irradiated with the light. Because the object to be measured 105 is a scattering medium, the signal light S reflected from the object may include the multiple scattered light waves having random phases. The signal light reflected from the object to be measured 105 and the reference light reflected from the mirror 102 to be subjected to the frequency shift are superimposed on each other by the beam splitter 103 to produce an interference light.
In the image measurement using the low coherent interference method, interference occurs only when a difference in optical path length between the signal light S and the reference light R is within the coherent length, of the broad band light source 101, which is of the order of several μm to tens of μm. In addition, only the component of the signal light S where phase is correlated to that of the reference light R may interfere with the reference light R. That is, only the coherent signal light component of the signal light S selectively interferes with the reference light R. Based on these principles, the position of the mirror 102 is shifted by the z-scanning operation to vary the optical path length of the reference light R, so that a light reflection profile of the inner layers of the object to be measured 105 is measured. The interference light is detected by the photo detector 104 during each z-scan. An electrical signal (heterodyne signal) output from the photo detector 104 provides a backscatter profile of the inner layers of object to be measured 105, and a two-dimensional cross-sectional image of the object to be measured 105 is produced by scanning the signal lights S across the object to be measured 105 while recording the reflectance profile at each transverse position (see Naohiro Tanno, Kogaku (Japanese Journal of optics), Volume 28, No. 3, 116 (1999)).
Assume that an intensity of the reference light R and an intensity of the signal light S which are superimposed by the beam splitter 103 are given by Ir and Is, respectively, and a frequency difference between the reference light R and the signal light S and a phase difference therebetween are given by fif and Δθ, respectively. In this case, a heterodyne signal as expressed by the following expression is outputted from the photo detector (for example, Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p. 2).i(t)∝Ir+Is+2√{square root over (IrIs)} cos(2πfift+Δθ)   (1)
The third term of the right side of the expression (1) indicates an alternating current electrical signal and the frequency fif thereof is equal to the frequency difference between the reference light R and the signal light S. The frequency fif of an alternating current component of the heterodyne signal is called a beat frequency or the like. The first and second terms of the right side of the expression (1) indicate the direct current components of the heterodyne signal and correspond to the background light intensity.
However, when the two-dimensional cross-sectional image is intended to be obtained by means of the conventional low coherent interference method, it is necessary to scan the signal light beam S across the object to be measured 105 and to successively detect reflection light waves from each transverse position. Therefore, the measurement of the object to be measured 105 can be time consuming. In addition, it is hard to shorten a measurement time in view of measurement fundamentals.
In views of such problems, an optical image measuring apparatus for shortening a measurement time has been proposed. FIG. 7 shows a fundamental structure of an example of such an apparatus. As shown in FIG. 7, an optical image measuring apparatus 200 includes a broad band light source 201, a mirror 202, a beam splitter (half mirror) 203, a two-dimensional photo sensor array 204 serving for light detection, and lenses 206 and 207. Alight beam from the light source 201 is converted into a parallel light flux by the lenses 206 and 207 and a beam diameter thereof is increased thereby. Then, the parallel light flux is divided by the beam splitter 203 into two, that is, the reference light R and the signal light S. The reference light R is subjected to a Doppler frequency shift by z-scanning of the mirror 202. On the other hand, the signal light S is incident on the object to be measured 205 over a wide area of the x-y plane, as a consequence of a widened beam diameter. Therefore, the signal light S reflected from the object to be measured 205 contains information related to the surface and inner portion of the object to be measured 205 over a wide area. The reference light R and the signal light S are superimposed on each other by the beam splitter 103 and detected by the elements (photo sensors) arranged in parallel on the surface of the two-dimensional photo sensor array 204. Thus, it is possible to obtain a two-dimensional cross-sectional image of the object to be measured 205 in real time without scanning the signal lights.
An apparatus described by K. P. Chan, M. Yamada, and H. Inaba in Electronics Letters, Vol. 30, 1753 (1994) has been known as such a non-scanning type optical image measuring apparatus. In the apparatus described in the same document, a plurality of heterodyne signals outputted from a two-dimensional photo sensor array are inputted to signal processing systems arranged in parallel to detect the amplitude and phase of each of the heterodyne signals.
However, when spatial resolution of an image is intended to be improved, it is necessary to increase the number of elements of the array. In addition, it is necessary to prepare a signal processing system including the number of channels corresponding to the number of elements. Therefore, it is likely to be hard to actually use the apparatus in fields that require a high-resolution image, such as a medical field and an industrial field.
Thus, the inventors of the present invention proposed the following non-scanning type optical image measuring apparatus in JP 2001-330558 A (claims, specification paragraphs [0068] to [0084], FIGS. 1 and 3). The optical image measuring apparatus according to the present proposal includes a light source for emitting a light beam, an interference optical system, and a signal processing portion. In the interference optical system, the light beam emitted from the light source is divided into two, that is, a signal light propagating through an examined object locating position in which an object to be examined is located, and a reference light propagating along an optical path different from an optical path passing through the examined object locating position. The signal light propagating through the examined object locating position and the reference light propagating along a different optical path are superimposed on each other to produce the interference light. The interference optical system includes a frequency shifter, light cutoff devices, and photo sensors. The frequency shifter shifts a frequency of the signal light and a frequency of the reference light relative to each other. In order to receive the interference light in the interference optical system, the interference light is divided into two parts. The light cutoff devices periodically cut off the two divided parts of the interference light to generate two interference light pulse trains with a phase difference of 90 degrees therebetween. The photo sensors respectively receive the two interference light pulse trains. The photo sensors each have a plurality of light receiving elements which are spatially arranged and each of which separately obtains a light receiving signal. The signal processing portion combines a plurality of light receiving signals obtained by each of the photo sensors to generate signals of the signal light which correspond to respective points of interest of a surface or inner layers of the object to be examined which is located in the examined object locating position on a propagation path of the signal light.
In the optical image measuring apparatus, the interference light in which the reference light and the signal light interfere with each other is divided into two parts and the two parts of the interference light are received by the two photo sensors (two-dimensional photo sensor arrays) and respectively sampled by the light cutoff devices disposed in fronts of both sensor arrays. A phase difference of π/2 is provided between sampling periods of the two divided parts of the interference light. Therefore, an intensity of the signal light and an intensity of reference light which compose background light of the interference light and phase quadrature components (sine component and cosine component) of the interference light are detected. In addition, an intensity of the background light included in the outputs from both the sensor arrays is subtracted from the outputs of both the sensor arrays to calculate two phase quadrature components of the interference light. An amplitude of the interference light is obtained based on a result obtained by calculation.
In the case of measurement using the optical image measuring apparatus described in JP 2001-330558 A, it is necessary to measure and obtain an intensity of a direct current component corresponding to the background light of the interference light included in the outputs from the two-dimensional photo sensor arrays in advance. Therefore, when the intensity of the direct current component can be measured, the availability of the measurement is shown. However, in the case where it is hard to measure the intensity of the direct current component corresponding to the background light, such as the case where an object to be measured, a part thereof, or an inner portion thereof is moving, it is difficult to measure the object to be measured using the optical image measuring apparatus.
An available image sensor such as a CCD (charge-coupled device) camera has been widely known as the two-dimensional photo sensor array of the optical image measuring apparatus. However, up to now, a problem has been recognized in that a currently available CCD camera cannot follow a beat frequency of a heterodyne signal which is of the order of several kHz to several MHz because of a low frequency response characteristic. The feature of the optical image measuring apparatus which is proposed by the inventors of the present invention and described in JP 2001-330558 A is that the measurement is performed using the low frequency response characteristic to advantage based on the sufficient recognition of the problem.